Heart disease is the leading cause of death in the United States. A heart attack (also known as an acute myocardial infarction (AMI)) typically results from a thrombus (i.e., a blood clot) that obstructs blood flow in one or more coronary arteries. AMI is a common and life-threatening complication of coronary artery disease. Coronary ischemia is caused by an insufficiency of oxygen to the heart muscle. Ischemia is typically provoked by physical activity or other causes of increased heart rate when one or more of the coronary arteries is narrowed by atherosclerosis. AMI, which is typically the result of a completely blocked coronary artery, is the most extreme form of ischemia. Patients will often (but not always) experience chest discomfort (angina) when the heart muscle is experiencing ischemia. Those with coronary atherosclerosis are at higher risk for AMI if the plaque becomes further obstructed by thrombus.
The current treatment for a coronary artery narrowing (a stenosis) is the insertion of a drug-eluting stent such as the Cypher™ sirolimus-eluting stent from Cordis Corporation or the Taxus™ paclitaxel-eluting stent from the Boston Scientific Corporation. The insertion of a stent into a stenosed coronary artery is a reliable medical treatment to eliminate or reduce coronary ischemia and to prevent the complete blockage of a coronary artery, which blockage can result in an AMI.
Acute myocardial infarction and ischemia may be detected from a patient's electrocardiogram (ECG) by noting an ST segment shift (i.e., voltage change). However, without knowing the patient's normal ECG pattern, detection from a standard 12 lead ECG can be unreliable.
Fischell et al. in U.S. Pat. Nos. 6,112,116, 6,272,379 and 6,609,023 describe implantable systems and algorithms for detecting the onset of acute myocardial infarction and providing both patient alerting and treatment. The Fischell et al. patents describe how the electrical signal from inside the heart (which is called an “electrogram”) can be used to determine various states of myocardial ischemia.
The Reveal™ subcutaneous loop Holter monitor sold by Medtronic, Inc., uses two case electrodes spaced about 3 inches apart to record electrocardiogram information. Recording can be triggered automatically when arrhythmias are detected or upon patient initiation using an external device. The Reveal is designed to record electrogram data only and does not include a patient alerting capability. The Reveal also does not have the capability to measure or alert the patient if there is an ST segment shift. In fact, the Reveal's high pass filtering and electrode spacing preclude accurate detection of changes in the low frequency aspects of the heart's electrical signal such as the ST segment of the electrogram.
While pacemakers and Implantable Cardioverter Defibrillators (ICDs) monitor the patient's electrogram, they do not currently detect ST segment changes nor provide patient alerting.
The term “medical practitioner” shall be used herein to mean any person who might be involved in the medical treatment of a patient. Such a medical practitioner would include, but is not limited to, a medical doctor (e.g., a general practice physician, an internist or a cardiologist), a medical technician, a paramedic, a nurse or an electrogram analyst. Although the masculine pronouns “he” and “his” are used herein, it should be understood that the patient, physician or medical practitioner could be a man or a woman. A “cardiac event” includes an acute myocardial infarction, ischemia caused by effort (such as exercise) and/or an elevated heart rate, bradycardia, tachycardia or an arrhythmia such as atrial fibrillation, atrial flutter, ventricular fibrillation, and premature ventricular or atrial contractions (PVCs or PACs respectively).
It is generally understood that the term “electrocardiogram” is defined as the heart's electrical signals sensed by means of skin surface electrodes that are placed in a position to indicate the heart's electrical activity (depolarization and repolarization). An electrocardiogram segment refers to a portion of electrocardiogram signal that extends for either a specific length of time, such as 10 seconds, or a specific number of heart beats, such as 10 beats. A beat is defined as a sub-segment of an electrogram or electrocardiogram segment containing exactly one R wave. As used herein, the PQ segment of a patient's electrocardiogram or electrogram is the typically straight segment of a beat of an electrocardiogram or electrogram that occurs just before the R wave and the ST segment is a typically straight segment that occurs just after the R wave.
Although often described as an electrocardiogram (ECG), the electrical signal from the heart as measured from electrodes within the body is properly termed an “electrogram”. As defined herein, the term “electrogram” is the heart's electrical signal voltage as sensed from one or more implanted electrode(s) that are placed in a position to indicate the heart's electrical activity (depolarization and repolarization). An electrogram segment refers to a portion of the electrogram signal for either a specific length of time, such as 10 seconds, or a specific number of heart beats, such as 10 beats. For the purposes of this specification, the terms “detection” and “identification” of a cardiac event have the same meaning.
A heart signal parameter is defined to be a measured or calculated value created during the processing of one or more beats of the electrogram (or electrocardiogram). Heart signal parameters include the following: ST deviation (ST segment average value minus PQ segment average value), ST shift (ST deviation compared to a baseline average ST deviation), average signal strength, T wave peak height, T wave average value, T wave deviation, QRS complex width, number of PVCs per unit time, heart rate and R-R interval.